Date of Award

8-2011

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Materials Science and Engineering

Advisor

Kennedy, M S

Committee Member

Ballato , J

Committee Member

Luo , J

Abstract

Nano-scale composites are being investigated by academic, industrial, and national laboratories for potential applications such as wear resistant coatings and high strength foils. These structures are of interest due to their as-fabricated mechanical properties, such as high strength for nanolaminates. However, there have only been limited studies of the long-term mechanical stability. A greater understanding of how these systems might respond to sustained use is needed before these systems transition from research laboratories into widespread application. It is the objective of this thesis to clarify the link between accelerated aging using elevated temperatures over time and mechanical properties of two distinct types of nano-scale composite systems: metallic nanolaminates and polyimide matrix nanocomposites.
Metallic nanolaminates are composites with alternating laminar metallic films whose individual layer thicknesses range from 2 nm to 100 nm. As the individual layer thicknesses decrease, the hardness of the nanolaminate system exceeds predictions from both conventional composite theories as well as the traditional Hall-Petch relation. This study examined how these nanolaminates responded to accelerated aging (elevated temperatures in three different atmospheres) by characterizing the mechanical response. Two different systems were studied, Cu/Nb, a FCC/BCC system studied by other groups, and Ti/W, a HCP/BCC system that has not been previously studied. Nanolaminate systems produced for this study had a total thickness of 1000 nm with individual layer thicknesses of 20 nm or 100 nm. After deposition, the film systems were heated to 400¡C for 30 minutes under lab air, high purity argon, or a 98%Ar/2%O2 blend. Accelerated aging of the Cu/Nb layers caused significant softening in the 20nm layer thickness samples (5.5 to 1.3 GPa for 20 nm Cu/Nb aged in Ar). Contrastingly, softening was not observed in 100 nm layer thickness samples (4.3 to 4.8 GPa for 100 nm Cu/Nb aged in Ar). This same trend was also followed by the other Cu/Nb samples and all the Ti/W systems regardless of aging atmosphere. Comparison with other studies show that residual stress may influence the severity of the film aging. It was concluded that for both systems oxidation is not the main concern for softening induced by accelerated aging.
The second type of nano-scaled composite system examined was polyimide matrix nanocomposites. NASA is exploring these systems for high-temperature aerospace applications, such as the matrix material for jet engine casings. This work looks at the mechanical aging of polyimide PMR-15, which is currently in use, and RTM-370, a possible replacement to PMR-15. In addition, this study looks at how the aging behavior is affected by the inclusion of three distinct nanoparticle filler materials: carbon nanofiber, synthetic clay, and organically modified natural clay. The elastic modulus of the polyimide nanocomposites were measured before and after accelerated aging. It was seen that the PMR-15 samples were susceptible to surface oxidation and stiffening due to the molding procedure (5.7 to 6.7 GPa for the edge region of neat PMR-15). Similar results were not found for the RTM-370 samples, as stiffness remained consistent regardless of indentation location. Nanoparticle addition to the polyimides yielded varying results. It is hypothesized that this disparity is due to the difference in particle surface area between nanoparticle fillers. Although the incorporation of nanoparticles was done in order to boost oxidation resistance in these systems, this was not observed in this study.

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